1. Field of the Invention
This invention relates generally to drill bits for use in drilling a borehole in a subterranean formation.
2. Background Art
Drill bits used to drill well bores through subterranean earth formations generally are made within one of two broad categories of bit structures. In the first category are the drill bits generally known as “roller cone” bits, which include a bit body having one or more roller cones rotatably mounted to the bit body. All known roller cone bits of prior art utilize cone or cylindrical bodies that rotate about a local axis that lies within a plane that is generally radial to the axis about which the drill bit rotates. The bit body is typically formed from steel or another high strength material. The roller cones are also typically formed from steel or other high strength material and include a plurality of teeth or cutting elements disposed at selected positions about the cones. The bit is secured to the lower end of a drill string that is rotated from the surface or by a down hole motor or turbine. The cutters mounted on the bit roll and slide upon the bottom of the borehole as the drill string is rotated, thereby engaging and crushing or disintegrating the formation material to be removed. The cutting elements on the rolling cutters are forced to penetrate and gouge the bottom of the borehole by weight of the drill string. The cuttings form the bottom and sides of the borehole are washed away by drilling fluid that is pumped down from the surface through the hollow, rotating drill string, and are carried in suspension in the drilling fluid to the surface.
Drill bits of the second category are typically referred to as “fixed cutter” or “drag” bits. A conventional fixed cutter drill bit has no moving elements but rather has a drill bit body typically having multiple blades, and cutters (sometimes referred to as cutter elements, cutting elements or inserts) attached at selected positions to the bit body or blades. The drilling mechanics and dynamics of a fixed cutter bit are different from those of roller cone bits. During drilling, fixed cutter bits are rotated against the subterranean formation being drilled under applied weight on bit to remove formation material. However, engagement between the cutting elements of a fixed cutter drill bit and the borehole bottom and sides shears or scrapes material from the formation, instead of using a crushing action as is employed by roller-cone bits.
The drill bit bodies to which cutting elements are attached in a fixed cutter drill bit may often be formed of steel or of molded tungsten carbide. Drill bit bodies formed of molded tungsten carbide (so-called matrix-type bit bodies) are typically fabricated by preparing a mold that embodies the inverse of the desired topographic features of the drill bit body to be formed. Examples of such topographic features include generally radially extending blades, sockets or pockets for accepting the cutting elements, junk slots, internal watercourses, nozzles and passages for delivery of drilling fluid to the bit face, ridges, lands, and the like. Tungsten carbide particles are then placed into the mold and a binder material, such as a metal including copper and tin, is melted or infiltrated into the tungsten carbide particles and solidified to form the drill bit body. Steel drill bit bodies, on the other hand, are typically fabricated by machining a piece of steel to form the desired external topographic features of the drill bit body. In both matrix-type and steel bodied drill bits, a threaded pin connection may be formed for securing the drill bit body to the drive shaft of a down hole motor or directly to drill collars at the lower end of a drill string rotated at the surface by a rotary table or top drive.
The cutting elements in a fixed cutter drill bit may be formed having a substrate or support stud made of carbide, for example tungsten carbide, and an ultra-hard cutting surface layer or “table” made of a polycrystalline diamond material or other super-abrasive material deposited onto or otherwise bonded to the substrate at an interface surface. One type of ultra-hard cutting surface for a fixed cutter bit is a layer of polycrystalline diamond formed on a substrate of tungsten carbide, typically known as polycrystalline diamond compact (PDC). Cutting elements are typically attached to matrix-type and steel bodied drill bits by either brazing or press-fitting the cutting elements into recesses or pockets formed in the bit face or in blades extending from the face. The cutting elements are attached to the bit bodies in this manner to ensure sufficient cutting element retention, as well as mechanical strength sufficient to withstand the forces experienced during drilling operations.
However, conventional fixed cutter drill bits having conventionally attached cutting elements suffer from a number of drawbacks and disadvantages. Because the cutting element is affixed to the bit body, only a portion of the circumferential cutting edge of the cutting element actually engages the subterranean formation being drilled. The constant engagement between this select portion of the cutting edge and the formation tends to quickly degrade and wear down the engaged portion of the cutting edge, resulting in decreased cutting element life, drilling efficiency, and accuracy. This constant engagement also significantly increases the temperature of the cutting element, which may further result in increased wear and/or potential destruction of the cutting element and drill bit body.
Many cutters develop cracking, spalling, chipping and partial fracturing of the ultra-hard material cutting layer at a region of cutting layer subjected to the highest loading during drilling. This region, often referred to as the “critical region,” encompasses the portion of the ultra-hard material layer that makes contact with the earth formations during drilling. The critical region is subjected to high magnitude stresses from dynamic normal loading, and shear loadings imposed on the ultra-hard material layer during drilling. Because the cutters are typically inserted into a drag bit at a rake angle, the critical region includes a portion of the ultra-hard material layer near and including a portion of the layer's circumferential edge that makes contact with the earth formations during drilling.
Additionally, another factor in determining the longevity of PDC cutters is the generation of heat at the cutter contact point, specifically heat generated from friction where the PDC layer is exposed to the formation. This heat causes thermal damage to the PDC in the fowl of cracks which lead to spalling of the polycrystalline diamond layer, delamination between the polycrystalline diamond and substrate, and back conversion of the diamond to graphite causing rapid abrasive wear. The high magnitude stresses at the critical region alone or in combination with other factors, such as residual thermal stresses, can result in the initiation and growth of cracks across the ultra-hard layer of the cutter. Cracks of sufficient length may cause the separation of a sufficiently large piece of ultra-hard material, rendering the cutter ineffective or resulting in the failure of the cutter. The high stresses, particularly shear stresses, may also result in delamination of the ultra-hard layer at the interface.
Bit designs in which one or more fixed cutters are individually rotatable are not an effective solution. An individual bit is subject to the same mechanical and thermal stresses described above. Further, it is very challenging to preserve the structural integrity of any small piece, such as an individual fixed cutter, under these extreme mechanical and thermal stresses.
In some fixed cutter bits, PDC cutters are fixed onto the surface of the bit such that a common cutting surface contacts the formation during drilling. Over time and/or when drilling certain hard but not necessarily highly abrasive rock formations, the edge of the working surface that constantly contacts the formation begins to wear down, forming a local wear flat, or an area worn disproportionately to the remainder of the cutting element. Local wear flats may result in longer drilling times due to a reduced ability of the drill bit to effectively penetrate the work material and a loss of rate of penetration caused by dulling of edge of the cutting element. That is, the worn cutter acts as a friction bearing surface that generates heat, which accelerates the wear of the cutter and slows the penetration rate of the drill. Such flat surfaces effectively stop or severely reduce the rate of formation cutting because the conventional cutters are not able to adequately engage and efficiently remove the formation material from the area of contact.
The failure conditions described above require expensive and time-consuming repair measures. Drilling operations may have to be ceased to allow for recovery and/or replacement of the drill bit and/or replacement of the ineffective or failed cutters.